The present invention relates to an apparatus and method for reconciling inventory with the volume of liquid dispensed from an above-ground storage tank (AST). In more detail, the present invention relates to a non-volumetric system for continuously monitoring the on-hand inventory in an AST and comparing the on-hand inventory to the amount of liquid that has been introduced into or dispensed from the AST at any given time to determine, for instance, the volume of liquid that has been lost from the AST or associated valves and piping, or through evaporation, and for providing inventory control.
Environmental awareness and concern for the economic loss from lost product has resulted in a relatively recent emphasis on insuring the tightness of liquid storage vessels. Of primary concern is the potential for contamination of underground water supplies caused by leaking gasoline or other refined petroleum products. In response to political pressures exerted by environmental and other interest groups, governmental authorities have imposed strict controls on the operation of such facilities to prevent contamination and to help arrest the deteriorating state of the environment.
Enforcement of these regulations has created a new and significant demand for testing procedures and equipment capable of detecting ever smaller amounts of leaking gasoline and other volatile organics. For instance, current United States governmental regulations specify that no leakage is allowable from an above ground storage tank and define a leak as a loss greater than 5 gallons per hour. It is expected that future standards will be even lower.
Most of the equipment and procedures currently known in the art for testing above ground storage tanks are not sufficiently accurate, and lack the greater resolution and precision required, to test against lower standards. Improved methods such as that described in U.S. Pat. No. 4,462,249 have made it possible to test underground storage tanks for leaks even smaller than 0.1 gal/hr, but so far as is known, no methods capable of such precision are available for testing above ground tanks for such leaks.
There are economic incentives to reduce the loss of product as well as the regulatory requirements. Lost product (through leakage or evaporation) cannot be sold to the consumer; further, the air pollution authorities of many states charge the operators of ASTs for evaporative losses. These charges are calculated from standard formulas based on the nature of the product in the tank, tank type, and tank geometry, but the formulas are only approximations of actual loss and may or may not accurately describe a particular AST. So far as is known, no method is currently available for measuring actual loss due to evaporation, and for those tanks with minimal evaporative loss, the operator could literally save thousands of dollars simply by demonstrating actual loss to the environmental authorities which charge such fees.
The difficulty with the testing of above ground tanks has been recognized for a long time. The diameter of such tanks is such that the drop in liquid level from a leak is barely perceptible because it is such a small portion of the total volume of the liquid in the tank. As noted in U.S. Pat. No. 3,062,994, in a tank having a diameter of two hundred feet, 10,000 gallons of liquid must be lost for the level to drop by 0.5 inches. Even if the capability existed to measure such volumes accurately, a 0.05 inch decrease in the liquid level (and assuming a linear correlation between liquid level and volume), would evidence the loss of 1,000 gallons, which is a leak which would be considered a major environmental incident by current criteria.
The patent literature evidences many attempts to overcome the difficulty of such volumetric measurements. For instance, U.S. Pat. No. 5,052,215 describes a method by which fluid is injected in the base beneath an above ground storage tank to enhance the rate of leakage from the bottom of the tank; leakage is then detected with acoustic sensors placed around the tank. Another approach is to use volatile liquid tracers as described in U.S. Pat. No. 5,048,324. The patent literature also includes grids or arrays of electronic probes (U.S. Pat. No. 4,646,069) for detecting the presence of the liquid that has leaked from the tank, ducts for conveying the gases from an escaped liquid past a sensor (U.S. Pat. No. 4,618,855), conductive wires which undergo a change in electrical properties when contacted by escaped liquid (U.S. Pat. No. 4,404,516), and reference and sensing electrodes for measuring changes in electric potential in the substrate/soil under the tank caused by escaped liquid (U.S. Pat. No. 4,166,244).
All of these methods described in the patent literature use some mechanism other than direct volumetric measurement of liquid loss to detect the leak and are, therefore, at best just an approximation of leak rate. Further, on information and belief, the results obtained with each of these methods, except perhaps with the method described in U.S. Pat. No. 5,048,324, will depend at least in part upon the amount of liquid in the tank. The hydrostatic pressure from the liquid in the tank varies with the depth of the liquid and it is this pressure, or weight, of the liquid which may cause a tank to leak when full; when the depth of the liquid decreases, the leak may decrease or even cease altogether.
Non-volumetric measurement is almost required of such tests because of the difficulty in detecting changes as small as those noted above in such large volumes of liquid. It is, therefore, a principal object of this invention to provide a non-volumetric method for detecting leaks which does not suffer from the disadvantages and limitations of these prior art methods. It does so by providing a method for detecting a change in the mass of the liquid in the tank resulting from a loss of the liquid through a leak. Mass measurement methods are known in the art. For instance, one method of measuring the mass of the fluid in a tank is to measure the pressure at the bottom of the tank, subtract the barometric pressure on the top of the tank and multiply this difference in pressure by the cross sectional area of the tank. This is an exact method only if the cross sectional area of the tank is uniform and does not change during the test.
To measure the differential pressure at the bottom of the tank, a differential pressure transducer located at the bottom of the tank must be used. This transducer must be able to survive in the environment at the bottom of the tank and must have a reference input from the top of the tank. The only pressure transducers capable of measurements with the required resolution must be exposed to a dry gas such as nitrogen, which is, of course, problematical if the environment in which pressure is being measured is submerged in a liquid. To overcome this difficultly with such mass measurement techniques, one must remove the transducer from the environment by using a bubbler system.
A bubbler system uses a bubble-forming tube which opens at the bottom of a fluid reservoir, a system to force bubbles out of the tube, and the pressure sensor. The bubbles require a particular pressure in order to be forced out of the tube in the bottom of the reservoir. This pressure is equal to the pressure produced by the fluid at the depth of the bubble formation.
Such systems are used for ascertaining and/or monitoring the depth of rivers and lakes in geographical surveying, and also are, on information and belief, used in such applications as monitoring the depth of liquid fuels in holding tanks at power generation facilities. Equipment for such known systems is available from, for instance, Uehling Instrument Company (Paterson, N.J.).
There is, however, a need for greater precision than can be obtained with the use of such known systems, and the present invention meets that need by providing an improvement to such systems which enables not only the detection, but also the quantification, of such leaks with a precision which meets and significantly exceeds applicable governmental regulatory requirements. Further, the present invention provides that information on a real-time basis, allowing the continuous comparison of the volume of liquid in an AST to the volume of liquid dispensed from the AST as obtained from the flow meter on the dispenser. The present invention has particular application to a gasoline storage and distribution facility because it is capable of providing this constant reconciliation of on hand inventory with the liquid volume dispersed from multiple ASTs, e.g., an AST for regular grade gasoline, premium, diesel, etc., but those skilled in the art who have the benefit of this disclosure will recognize that the invention has broad applicability to an liquid storage container which receives and dispenses liquid.